fix(router): allow surplus from swap when routing intents (#38)

* fix(router): allow surplus from swap when routing intents

* fmt

Author: lumtis

src/Router.sol

@@ -389,11 +389,13 @@ contract Router is
                 zrc20ToTokenName[intentInfo.zrc20]
             );
 
-            // Validate that swap result is not greater than expected amount
-            require(wantedAmountWithTip >= settlementInfo.amountWithTipOut, "Swap returned invalid amount");
-
             // Calculate slippage difference and adjust tip accordingly
-            uint256 slippageAndFeeCost = wantedAmountWithTip - settlementInfo.amountWithTipOut;
+            // If swap returns more than expected (surplus), slippageAndFeeCost will be 0
+            // Note: Surplus amounts are currently ignored as they are too small to handle
+            // This will be addressed in future updates
+            uint256 slippageAndFeeCost = wantedAmountWithTip > settlementInfo.amountWithTipOut
+                ? wantedAmountWithTip - settlementInfo.amountWithTipOut
+                : 0;
 
             // Check if tip covers the slippage and fee costs
             if (wantedTip > slippageAndFeeCost) {

test/Router.t.sol

@@ -1392,7 +1392,7 @@ contract RouterTest is Test {
         // even better with an explicit check of the settlement payload
     }
 
-    function test_OnCall_InvalidSwapAmountOut() public {
+    function test_OnCall_SwapSurplusHandling() public {
         // Setup intent contract
         uint256 sourceChainId = 1;
         uint256 targetChainId = 2;
@@ -1421,7 +1421,7 @@ contract RouterTest is Test {
         // Set modest slippage (5%)
         swapModule.setSlippage(500);
 
-        // Set a custom amount out for testing that is more than the wanted amount
+        // Set a custom amount out for testing that is more than the wanted amount (surplus)
         swapModule.setCustomAmountOut(amount + tip + 1 ether);
 
         // Mock setup for IZRC20 withdrawGasFeeWithGasLimit
@@ -1446,10 +1446,11 @@ contract RouterTest is Test {
             senderEVM: sourceIntentContract
         });
 
-        // Call onCall
+        // Call onCall - should succeed even with surplus
         vm.prank(address(gateway));
-        vm.expectRevert("Swap returned invalid amount");
         router.onCall(context, address(inputToken), amount + tip, intentPayloadBytes);
+
+        // Test passes if no revert occurs - surplus is handled gracefully
     }
 
     function test_OnCall_UsesCustomGasLimitFromPayload() public {